The height of the "panelmultiview" binding is now determined by the stack layout code, and doesn't have to be calculated manually via JavaScript anymore. This allows the removal of mutation and overflow observers, and reduces the number of synchronous layouts being made.
There is still a workaround included for wrapping blocks not being taken into account in height calculations.
MozReview-Commit-ID: 9rrPU5O5hUx
efed284f45